The present invention principally relates to an electric dust collector for cleaning the air inside a room.
Heretofore, as a conventional electric dust collector for cleaning the indoor air, there has been used a two-stage system electric dust collector equipped with a charging section for electrically charging the dust particles using corona discharge and a collecting section for collecting the dust particles by means of the Coulomb force consisting of positive and negative parallel flat electrode plates. In addition to its complicated construction and its high cost of production, this electric dust collector is extremely difficult to reduce the apparatus in size and cannot enhance its dust collecting performance by making narrower the gaps between the positive and negative electrodes of the dust collecting section.
This arises from the fact that when the electrode gap is markedly reduced, insulation becomes difficult.
In addition, the conventional electric dust collector is not free from the following problems. Namely, when it is desired to clean the air containing odor particles such as tobacco smoke by the conventional electric dust collector, an offensive odor remains in the particle collecting section and is then carried over by the cleaned air. Though it is therefore desired to replace the particle collecting zone, the apparatus is so constructed that the particle collecting section is not replacable and even if it is possible, the particle collecting section is expensive or difficult to replace. Hence, one has to wait till the residual offensive odor disappears naturally with the passage of time.
Further, the particle collecting section of the conventional electric dust collector involves another problem in that if the gap between the electrodes is kept narrower than the quenching distance (that is, a distance that prevents ignition of an inflammable gas coming into the electrode gap when spark occurs), it is certainly possible to expect the quenching effect and thus to prevent the ignition, but when the spark occurs in the proximity of the electrode plates, the quenching effect is not provided and the danger of ignition is therefore present.
Hereinafter, these problems will be explained more definitely.
The conventional particle collecting section is shown in FIG. 1 wherein a corrugated insulation spacer 3 is interposed between and over the entire surfaces of electrically conductive electrode plates 1 and 2 or such a construction as shown in FIG. 2 wherein one of, or both, electrode plates 1 and 2 is covered with an insulation material 4. However, these constructions have the following disadvantages, respectively.
As to the former:
I. The charged particles are to pass either through the space S or the space S' of the particle collecting section. When the particles 5 charged with a positive charge pass through the space S, they are subjected to the Coulomb force arising from the electric field formed by a voltage V applied across the electrode plates 1 and 2, so that the particles travel towards the negative electrode plate 2 and attach to the surfaces of the spacer 3 as depicted in FIG. 3. On the other hand, when the particles 6 charged with a negative charge pass through the space S', they also are subjected to the Coulomb force so that they travel toward the positive electrode plate 1 and attach to the surfaces of the spacer 3 as depicted in FIG. 4. In this instance, since the spacer 3 is made of an insulation material, the charges of the particles 5 and 6 stay on the surfaces of the spacer 3 and build-up thereon whereby the electric field generated by themselves offsets the electric field applied across the electrode plates 1 and 2 and the electric field of either the space S or of the space S' is weakened.
When the electrically conductive particles such as nicotine and tobacco tar smoke attach to the surfaces of the spacer 3 and build up gradually thereon, electrically conductive layers 7 and 8 are formed on both surfaces of the spacer 3, so as to create a situation similar to that where the electrode plate 1 extends up to the conductive layer 7 inside the space S while the electrode plate 2 extends up to the conductive layer 8 inside the space S'. In consequence, no electric field is given to the space S or to the space S' even when a voltage V is applied across the electrode plates 1 and 2.
For this reason, the dust collecting efficiency lowers with the lapse of usage time and the service life of the dust collector is short, although the collector provides good dust collecting efficiency at the initial stage.
II. Since the spacer 3 is interposed between and over the entire surfaces of the electrode plates 1 and 2, a leakage current flowing from one of the electrode plates to the other through the surfaces of the spacer 3 is great. Especially when the humidity is high, contaminants attaching to the surfaces of the spacer 3 absorb the moisture and enlarge the conductivity, thus resulting in the drop of the power source voltage and lowering of the dust colllecting efficiency. Hence, practical utility is poor.
III. If the capacity of the particle collecting section is the same, it is possible to increase the number of electrode plates and thus to enhance the dust collecting efficiency by narrowing the gap between the electrode plates 1 and 2. If the number of the electrode plates remains the same, it is possible to reduce the capacity of the particle collecting section and hence, to reduce the size of the electric dust collector. In spite of these advantages, the electric field tends to concentrate on the edge portions of the electrode plates 1 and 2 or fiber-like dusts tend to bridge the electrode plates 1, 2 so that the spark will occur more readily. This leads to various problems such that application of a high voltage becomes impossible; an offensive noise occurs; and the electrode plates are scorched or melted and thus degraded, thereby resulting in causes for possible fire. Accordingly, it is practically impossible to narrow the gap between the electrode plates and consequently, to render the electric dust collector more compact.
IV. In order to narrow the electrode gap and at the same time, to prevent occurrence of the spark, the following methods may be theoretically possible, but each is not free from the peculiar problems.
(i) A method which removes in advance coarse or fiber-like dusts by use of a pre-filter.
In order to efficiently remove dust by use of a pre-filter having a fine mesh in accordance with this method, it is necessary to apply a high pressure so as to pass the air through the electric dust collector. To meet with this requirement, the use of a large-sized blower becomes necessary and there again occur other problems such as the increase in noise, the consumption of a greater electric power and so forth. In other words, if a pre-filter having a large mesh is used in order to minimize the pressure loss, it becomes impossible to sufficiently remove the dust and hence, to prevent the occurrence of the spark.
(ii) A method which lowers the voltage to such a level where no spark occurs so as to weaken the electrolysis of the particle collecting section.
This method has the drawback in that the dust collecting efficiency is inevitably lowered.
(iii) A method which covers the edge of the electrode plate with an insulation material.
This method contemplates preventing the ready occurrence of a spark as the electric field concentrates on the edge of the electrode plate. However, the spark occurs not only at the edge of the electrode plate but also at its central portion. This tendency is remarkable when the fiber-like dust attaches to the surface. In such a case, this method becomes practically useless.
(iv) A method which covers both, or one, of the positive and negative electrode plates with an insulation material.
Since the electric field or voltage in the insulating material at which no spark occurs is about 10 times higher than that of the air, this method is certainly useful for the prevention of the occurence of a spark, but it is not free from the problem of the build-up of the charge.
As to the latter:
Since the electrode plate is covered with the insulation material 4, it is possible to prevent the occurrence of the spark in the same way as in the abovementioned method (iv). However, when the charged particles 5 attach to the insulation material 4 as depicted in FIG. 2, the electric charge does not escape but stays on the surfaces of the insulation material, thereby weakening the electric field. Since this latter construction is also provided with the spacer 3 of any sort as shown in FIG. 6, the dusts attaching to the surfaces of the spacer, if any, form the conductive layers, thereby eliminating the electric fields of the spaces S and S'.